The invention relates to a display device comprising a display panel with a first light-transmitting substrate provided with reflective material, a second light-transmitting substrate and an electro-optical material between said two substrates.
Such display devices are used, for example, in (portable) display screens in hand-held telephones, organizers but also, for example, in automotive applications.
A (transfective) display device of the type mentioned in the opening paragraph is described in IBM TDB Vol. 15, No. 8, pp. 2435-6. In the reflective state, ambient light is reflected by the reflective material, in this case a reflective electrode (a partly covered mirror) of, for example, chromium or aluminum. In the transmissive state, these electrodes pass light, and in the reflective state they reflect incident light. The actual picture elements (characters) are provided on the electrodes by means of etching.
To ensure that sufficient light can be passed in the transmissive state, the mirror must not be thick (in the case of aluminum, for example, thinner than 15 nm). It is very difficult to provide such mirrors with sufficient accuracy. Variations in thickness cause large variations in light transmission and, as a result, lead to non-uniform behavior in both the reflective state and the transmissive state. In the case of relatively large panels, the small thickness additionally influences the drive behavior because the square resistance becomes too high.
Another problem arises if birefringent material, for example twisted nematic (liquid-crystal) material is used in such a display device, because said material causes the transmission-voltage curve to be different in the transmissive mode and in the reflective mode.